Update

You need to use --enable-databasedesigner with ./configure to enable
it. By default, it's disabled.

There are too many bugs and rough edges and not enough people interested to work on it.

pgAdmin had a GSoC project in 2011, in the course of which Luis Ochoa designed the new database designer - which can also reverse engineer ER diagrams from existing dbs. Unfortunately it wasn't ready for release with pgAdmin 1.14 (the current version). But it's in the repository for version 1.16 that will probably accompany the release of PostgreSQL 9.2.